Expandable core for use in casting

ABSTRACT

A salt core ( 2 ) is formed by casting a mixed material of a salt material and a ceramic material. Any one of a chloride, bromide, carbonate, and sulfate of potassium or sodium is used as the salt material. As the ceramic material, granular one having a density falling within a range of 2.2 g/cm 3  (exclusive) to 4 g/cm 3  (inclusive) is used.

TECHNICAL FIELD

The present invention relates to expendable salt-core for use incasting, which is loaded in a mold used for forming non-ferrous alloycastings, particularly a high pressure die-casting mold as well, canwithstand a high casting pressure environment, and is formed from a saltmaterial.

BACKGROUND ART

Conventionally, high pressure die-casting can afford to manufacture withhigh volume production for complicated-shape components with highdimensional accuracy at a low cost. Although, depending on the shaperestriction of the components, an expendable core for use in casting mayhave to be used. Conventionally, as an expendable core, in addition toexpendable sand cores formed using sand, a so-called salt core isavailable. The salt core is a very attractive choice in the light of theproductivity.

More specifically, after casting process is finished, the salt core canbe removed by dissolving it with hot water or steam. When the salt coreis used, as compared to a case wherein a sand core (e.g., a shell moldcore) is used, cumbersome sand removing operation can be eliminated toimprove the productivity. With a sand core, chiefly because a so-calledmetal penetration phenomenon occurs, that is, the melt enters gaps amongsand grains in the boundary with the core and accordingly the sandcannot be easily removed.

Therefore, after the product is extracted from the mold, the productmust be subjected to several knock-out machines to discharge the sand inthe product. Furthermore, sand that does not fall readily due to metalpenetration must be dropped by shot blasting. Hence, the sand removingoperation is cumbersome, leading to an increase in cost.

A salt core of this type is formed from sodium chloride (NaCl) orpotassium chloride (KCl) as a main material (salt material), asdisclosed in, e.g., Japanese Patent Publication No. 48-17570 (to bemerely referred to patent reference 1 hereinafter), U.S. Pat. No.3,963,818 (to be merely referred to as patent reference 2 hereinafter),U.S. Pat. No. 4,361,181 (to be merely referred to as patent reference 3hereinafter), and U.S. Pat. No. 5,165,464 (to be merely referred to aspatent reference 4 hereinafter).

The salt core shown in each of patent references 1 to 3 is formed bymolding a chloride such as granular (powder) sodium chloride orpotassium chloride into a predetermined shape by press molding andsintering the molded material.

The salt core described in patent reference 4 uses sodium chloride asthe salt material and is molded into a predetermined shape bydie-casting.

Each of U.S. Pat. No. 4,446,906 (to be merely referred to as patentreference 5 hereinafter), U.S. Pat. No. 5,803,151 (to be merely referredto as patent reference 6 hereinafter), Japanese Patent Publication No.49-15140 (to be merely referred to patent reference 7 hereinafter),Japanese Patent Publication No. 48-8368 (to be merely referred to aspatent reference 8 hereinafter), Japanese Patent Publication No.49-46450 (to be merely referred to as patent reference 9 hereinafter),and U.S. Pat. No. 4,840,219 (to be merely referred to as patentreference 10 hereinafter) discloses a salt core in which ceramic ismixed as a filler in the salt material.

The salt core shown in patent reference 5 uses silica (SiO₂) or alumina(Al₂O₃) as reinforcement and is molded into a predetermined shape bydie-casting. The tensile strength of the salt core is described as 150psi to 175 psi which corresponds to 1.03 MPa to 1.2 MPa. With a sandcore which is also a expendable core, the strength of the core isgenerally evaluated from the value of the bending strength obtained by abending strength test. With the salt core as well, an evaluating methodusing the bending strength can be employed.

The bending strength is a barometer that indicates the strength of anexpendable core when a bending stress acts on the expendable core. Abending stress supposedly acts, for example, when a melt flows from agate into a mold cavity at a high speed to collide against an internalsalt core, or when an impact acts on a core as the core is beingattached in a mold. The bending stress which is generated in this manneris the main factor that breaks the core in high pressure die-casting ata high speed injection. Patent reference 5 has no description on thebending stress. Although the specification of patent reference 5describes that an engine block is produced by die-casting using the saltcore, it has no commercial record. Therefore, it is estimated that thesalt core did not have a bending stress that could stand the high meltand high injection speed of high pressure die-casting.

The salt core shown in patent reference 6 uses particles, fibers, andwhiskers of alumina, silica sand, boron nitride (BN), boron carbide(BC), as reinforcement. The salt core is formed by molding a mixture ofthe reinforcement and salt material into a predetermined shape bypressurized molding and sintering the molded material. This patentsuggests that the salt core is reinforced by various types of ceramics,although the process is different.

The salt core shown in each of patent references 7 and 8 uses alumina asreinforcement. The salt core shown in patent reference 9 uses silica,alumina, zirconia (ZrO₂), or the like as reinforcement. The salt coresshown in patent references 7 to 9 are formed by casting.

The salt core shown in patent reference 10 is formed by mixing two typesof alumina having different particle sizes as reinforcement in a saltmaterial and molding the mixture into a predetermined shape bydie-casting. The salt material used for the salt core is a mixed saltobtained by mixing sodium carbonate (Na₂CO₃) in sodium chloride.

A salt core which uses a mixed salt as the salt material in this manneris also described in U.S. Pat. No. 5,303,761 (to be merely referred toas patent reference 11 hereinafter) and Japanese Patent Laid-Open No.50-136225 (to be merely referred to as patent reference 12 hereinafter)in addition to the above patent references.

Patent reference 11 shows a mixed salt which is made from sodiumchloride and sodium carbonate in the same manner as in patent reference10. Patent reference 12 discloses a mixed salt obtained by mixingpotassium chloride and sodium chloride in sodium carbonate.

A salt material obtained by mixing ceramic in a mixed salt is shown inJapanese Patent Publication No. 48-39696 (to be merely referred to aspatent reference 13 hereinafter) and Japanese Patent Laid-Open No.51-50218 (to be merely referred to as patent reference 14 hereinafter).

Patent reference 13 shows a salt material obtained by mixing a metaloxide such as alumina or zinc oxide (ZnO) and a siliceous granularmaterial such as silica sand, talc, or clay in a mixed salt made fromsodium carbonate, sodium chloride, and potassium chloride.

Patent reference 14 shows a salt material obtained by mixing silica,alumina, fiber, or the like in a mixed salt made from potassiumcarbonate, sodium sulfate (Na₂SO₄), sodium chloride, and potassiumchloride.

When a salt material is used as a mixed salt in this manner, the meltingpoint of the salt material can be relatively decreased more as comparedwith a case wherein the salt material is made from a single typechloride, carbonate, or sulfate.

DISCLOSURE OF INVENTION

Problem to be Solved by the Invention

The salt core shown in each of patent references 1 to 3 and 6 describedabove is formed by press molding and accordingly cannot be formed into acomplicated shape. This problem can be solved to a certain degree byforming the salt core by casting such as die-casting, as shown in patentreferences 4, 5, 10, and 11. The salt core shown in patent reference 4,however, has a low bending strength. When a product is to be cast usingthis salt core, limitations and conditions in casting increase.

More specifically, in the salt core shown in patent reference 4, thematerial itself of the core is made from a brittle material (e.g., witha bending strength of 1 MPa to 1.5 MPa) such as sodium chloride orpotassium chloride. Hence, this core can only be used in, e.g.,parmanent mold casting or low pressure die casting (LP) in which themelt supply pressure is low and the melt flow rate is suppressed so thecore will not be damaged during product casting, and cannot be used inhigh pressure, high speed die-casting generally called die-casting.Conventional die-casting requires a higher melt pressure of 40 MPa to100 MPa during casting and a higher injection speed (a gate rate of 20m/sec to 100 m/sec) than in permanent mold casting and low pressure diecasting. Even a core different from a salt core is difficult to use inconventional die-casting. In laminar flow die-casting, squeezedie-casting, or the like in which the melt supply pressure is high butthe supply rate is low, a shell core {with a bending strength of 3 MPato 6 MPa (the present maximum value: 6 MPa)} with an improved strengthmay be used. In this case, however, the time required for sand removalafter casting becomes excessively long, and the manufacturing costincreases greatly.

In order to increase the bending strength of the salt core, ceramic maybe mixed as a reinforcing material in the salt material, as shown inpatent references 5, 10, 13, and 14. With a conventional ceramic-mixedsalt core; however, a high expected bending strength cannot be obtained.This may be due to the following reasons. A versatile industrialmaterial or natural material (e.g., alumina or silica) may be mainlyused as the ceramic material, and accordingly the ceramic material maynot sufficiently disperse in the salt material. Alternatively, a ceramicmaterial having appropriate physical properties may not be used.

The present invention has been made to solve the above problem, and hasas its object to provide a salt core which has high fluidity, can beformed into a core with a complicated shape by casting such asdie-casting, parmanent mold-casting, and low pressure die casting, has ahigh bending strength as a core, and can be applied to die-casting aswall.

In recent years, artificially synthesized ceramic or the like (which maybe obtained by remelting, grinding, and classifying kaolin and may be aground product of, e.g., synthetic mullite; may be obtained bygranulating, sintering with a rotary kiln, and classifying kaolin andmay be a sintered product of, e.g., synthetic mullite; may be obtainedby sedimentation by the flux scheme, removing flux, and classificationand may be, e.g., aluminum borate; or may be obtained by sedimentationby vapor deposition and classification and may be, e.g., silicon carbideor silicon nitride) has been under production.

These artificially synthesized materials are conventionally used as areinforcing material for a reinforced plastic material, as aheat-resistant piston material, in a break shoe as an alternativematerial to asbestos, or as an industrial material developed foraviation and space technology. None of the artificially synthesizedmaterials is developed as salt core reinforcing ceramic.

Such artificially synthesized materials are marketed with variousdensities, particle sizes, shapes, and the like, and their heatresistances and strength stabilities are greatly improved over those ofconventional ceramic. In view of this fact, the present inventorsre-examined the possibility of these materials as salt-reinforcingceramic materials, and reached the present invention.

Means of Solution to the Problem

In order to achieve the above object, according to the presentinvention, there is provided a core for use in casting which is formedby casting a mixed material of a salt material and a ceramic material,the salt material comprising any one of a chloride, a bromide, acarbonate, and a sulfate of any one of potassium and sodium, and theceramic material comprising artificially synthesized granular one havinga density falling within a range of 2.2 g/cm³ (exclusive) to 4 g/cm³(inclusive).

According to claim 2 of the present invention, there is provided a corefor use in casting according to claim 1 of the present invention,wherein the ceramic material comprises synthetic mullite having adensity of 2.79 g/cm³ to 3.15 g/cm³.

According to claim 3 of the present invention, there is provided a corefor use in casting according to claim 1, wherein the ceramic materialcomprises aluminum borate having a density of 2.93 g/cm³.

According to claim 4 of the present invention, there is provided a corefor use in casting which in formed by casting a mixed material of a saltmaterial and a ceramic material, the salt material comprising any one ofa chloride, a bromide, a carbonate, and a sulfate of any one ofpotassium and sodium, and the ceramic material comprising artificiallysynthesized granular one having a particle size of not more than 150 μm.

According to claim 5 of the present invention, there is provided a corefor use in casting which is formed by casting a mixed material of a saltmaterial and a ceramic material, said salt material comprising any oneof a chloride, a bromide, a carbonate, and a sulfate of any one ofpotassium and sodium, and said ceramic material comprising any granularone of synthetic mullite, aluminum borate, boron carbide, siliconnitride, silicon carbide, aluminum nitride, aluminum titanatecordierite, and alumina.

According to claim 6 of the present invention, there is provided a corefor use in casting which is formed by casting a mixed material of a saltmaterial and a ceramic material, the salt material comprising any one ofa chloride, a bromide, a carbonate, and a sulfate of any one ofpotassium and sodium, and the ceramic material comprising whiskers ofany one of aluminum borate, silicon nitride, silicon carbide, potassiumhexatitanate, potassium octatitanate, and zinc oxide.

According to claim 7 of the present invention, there is provided a corefor use in casting according to claim 6 of the present invention,wherein the ceramic material comprises aluminum borate whiskers.

According to claim 8 of the present invention, there is provided a corefor use in casting which is formed by casting a mixed material of a saltmaterial and a ceramic material, the salt material comprising a mixedsalt obtained by adding any one of a carbonate and a sulfate of any oneof potassium and sodium to a chloride of any one of potassium andsodium, and the ceramic material comprising artificially synthesizedgranular one having a density falling within a range of 2.2 g/cm³(exclusive) to 4 g/cm³ (inclusive).

According to claim 9 of the present invention, there is provided a corefor use in casting according to claim 8 of the present invention,wherein the ceramic material comprises synthetic mullite having adensity falling in a range of 2.79 g/cm³ to 3.15 g/cm³.

According to claim 10 of the present invention, there is provided a corefor use in casting according to claim 8 of the present invention,wherein the ceramic material comprises aluminum borate having a densityof 2.93 g/cm³.

According to claim 11 of the present invention, there is provided a corefor use in casting which is formed by casting a mixed material of a saltmaterial and a ceramic material, the salt material comprising a mixedsalt obtained by adding any one of a carbonate and a sulfate of any oneof potassium and sodium to a chloride of any one of potassium andsodium, and the ceramic material comprising artificially synthesizedgranular one having a particle size of not more than 150 μm.

According to claim 12 of the present invention, there is provided a cotefor use in casting which is formed by casting a mixed material of a saltmaterial and a ceramic material, the salt material comprising a mixedsalt obtained by adding any one of a carbonate and a sulfate of any oneof potassium and sodium to a chloride of any one of potassium andsodium, and the ceramic material comprising any granular one ofsynthetic mullite, aluminum borate, boron carbide, silicon nitride,silicon carbide, aluminum nitride, aluminum titanate, cordierite, andalumina.

According to claim 13 of the present invention, there is provided a corefor use in casting which is formed by casting a mixed material of a saltmaterial and a ceramic material, the salt material comprising a mixedsalt obtained by adding any one of a carbonate and a sulfate of any oneof potassium and sodium to a chloride of any one of potassium andsodium, and the ceramic material comprising whiskers of any one ofaluminum borate, silicon nitride, silicon carbide, potassiumhexatitanate, potassium octatitanate, and zinc oxide.

According to claim 14 of the present invention, there is provided a corefor use in casting according to claim 13 of the present invention,wherein the ceramic material comprises aluminum borate whiskers.

According to claim 15 of the present invention, there is provided a corefor use in casting according to any one of claims 8 to 14 of the presentinvention, wherein the mixed salt is made from potassium chloride andsodium carbonate.

Effect of the Invention

As has been described above, according to the present invention, a saltcore in which a ceramic material sufficiently disperses in a saltmaterial can be formed by casting.

Therefore, a core for use in casting according to the present inventioncan be formed into a complicated shape by casting while having suchcharacteristics that it can be removed by water (including hot water orsteam) after casting, and its bending strength is increased more thanexpected by a reinforcing material made from a ceramic material. Hence,the core for use in casting according to the present invention can alsobe used in, e.g., a die cast machine which is conventionally difficultto use it. Moreover, when mounting the core in another matrix, the coreneed not be handled particularly carefully. Thus, the degrees of freedomof casting can be increased.

According to claim 2 of the present invention, a salt core in whichsynthetic mullite sufficiently disperses in a salt material can beformed by casting.

According to claim 3 of the present invention, a salt core in whichaluminum borate sufficiently disperses in a salt material can be formedby casting.

According to claim 4 of the present invention, a salt core in which asalt material sufficiently disperses in a salt material can be formed bycasting.

Therefore, a core for use in casting according to the present inventioncan be formed into a complicated shape by casting while having suchcharacteristics that it can be removed by water (including hot water orsteam) after casting, and its bending strength is increased more thanexpected by a reinforcing material made from a ceramic material. Hence,the core for use in casting according to the present invention can alsobe used in, e.g., a die cast machine which is conventionally difficultto use it. Moreover, when mounting the core in another matrix, the coreneed not be handled particularly carefully. Thus, the degrees of freedomof casting can be increased.

According to claim 5 of the present invention, a salt core which issufficiently reinforced by a granular ceramic material can be formed.

Therefore, a core for use in casting according to the present inventioncan be formed into a complicated shape by casting while having suchcharacteristics that it can be removed by water (including hot water orsteam) after casting, and its bending strength is increased more thanexpected by a reinforcing material made from a granular ceramicmaterial. Hence, the core for use in casting according to the presentinvention can also be used in, e.g., a die cast machine which isconventionally difficult to use it. Moreover, when mounting the core inanother matrix, the core need not be handled particularly carefully.Thus, the degrees of freedom of casting can be increased. As one type ofceramic material is used, the salt core can be dissolved in water torecover the ceramic material, so that the ceramic material can berecycled.

According to claim 6 of the present invention, a salt core which issufficiently reinforced by whiskers made from a ceramic material can beformed.

Therefore, a core for use in casting according to the present inventioncan be formed into a complicated shape by casting while having suchcharacteristics that it can be removed by water (including hot water orsteam) after casting, and is sufficiently reinforced by the whiskersmade from a ceramic material, so that its bending strength is increasedmore than expected. Hence, the core for use in casting according to thepresent invention can also be used in, e.g., a die cast machine which isconventionally difficult to use it. Moreover, when mounting the core inanother matrix, the core need not be handled particularly carefully.Thus, the degrees of freedom of casting can be increased. As one type ofceramic material is used, the salt core can be dissolved in water torecover the ceramic material, so that the ceramic material can bereused.

According to claim 7 of the present invention, a salt core which issufficiently reinforced by aluminum borate whiskers can be formed bycasting.

According to claim 8 of the present invention, a salt core in which aceramic material sufficiently disperses in a salt material made from amixed salt can be formed by casting.

Therefore, a core for use in casting according to the present inventioncan be formed into a complicated shape by casting while having suchcharacteristics that it can be removed by water (including hot water orsteam) after casting, and its bending strength is increased more thanexpected by a reinforcing material made from a ceramic material. Hence,the core for use in casting according to the present invention can alsobe used in, e.g., a die cast machine which is conventionally difficultto use it. Moreover, when mounting the core in another matrix, the coreneed not be handled particularly carefully. Thus, the degrees of freedomof casting can be increased.

The salt material of the salt core is a mixed salt and its melting pointdecreases relatively. Hence, the temperature required when casting thesalt core can be decreased, and the manufacturing cost of the salt corecan be decreased. Also, a salt core with small unevenness formed on acore surface can be provided.

According to claim 9 of the present invention, a salt core in whichsynthetic mullite sufficiently disperses in a salt material made from amixed salt can be formed by casting.

According to claim 10 of the present invention, a salt core in whichaluminum borate sufficiently disperses in a salt material made from amixed salt can be formed by casting.

According to claim 11 of the present invention, a salt core in which aceramic-material sufficiently disperses in a salt material made from amixed salt can be formed by casting.

Therefore, a core for use in casting according to the present inventioncan be formed into a complicated shape by casting while having suchcharacteristics that it can be removed by water (including hot water orsteam) after casting, and its bending strength is increased more thanexpected by a reinforcing material made from a ceramic material. Hence,the core for use in casting according to the present invention can alsobe used in, e.g., a die cast machine which is conventionally difficultto use it. Moreover, when mounting the core in another matrix, the coreneed not be handled particularly carefully. Thus, the degrees of freedomof casting can be increased.

The salt material of the salt core is a mixed salt, and its meltingpoint decreases relatively. Hence, the temperature required when castingthe salt core can be decreased, and the manufacturing cost of the saltcore can be decreased. Also, a salt core with small unevenness formed ona core surface can be provided.

According to claim 12 of the present invention, a salt core in which agranular ceramic material sufficiently disperses on a salt material madefrom a mixed salt and which is sufficiently reinforced by the ceramicmaterial can be formed.

Therefore, a core for use in casting according to the present inventioncan be formed into a complicated shape by casting while having suchcharacteristics that it can be removed by water (including hot water orsteam) after casting, and its bending strength is increased more thanexpected by a reinforcing material made from a granular ceramicmaterial. Hence, the core for use in casting according to the presentinvention can also be used in, e.g., a die cast machine which isconventionally difficult to use it. Moreover, when mounting the core inanother matrix, the core need not be handled particularly carefully.Thus, the degrees of freedom of casting can be increased.

The salt material of the salt core is a mixed salt, and its meltingpoint decreases relatively. Hence, the temperature required when castingthe salt core can be decreased, and the manufacturing cost of the saltcore can be decreased. Also, a salt core with small unevenness formed ona core surface can be provided.

According to claim 13 of the present invention, a salt core in whichceramic whiskers sufficiently disperse in a salt material made from amixed salt and which is sufficiently reinforced by the whiskers can beformed.

Therefore, a core for use in casting according to the present inventioncan be formed into a complicated shape by casting while having suchcharacteristics that it can be removed by water (including hot water orsteam) after casting, and its banding strength is increased more thanexpected by a reinforcing material made from a granular ceramicmaterial. Hence, the core for use in casting according to the presentinvention can also be used in. e.g., a die cast machine which isconventionally difficult to use it. Moreover, when mounting the core inanother matrix, the core need not be handled particularly carefully.Thus, the degrees of freedom of casting can be increased.

The salt material of the salt core is a mixed salt, and its meltingpoint decreases relatively. Hence, the temperature required when castingthe salt core can be decreased, and the manufacturing cost of the saltcore can be decreased. Also, a salt core with small unevenness formed ona core surface can be provided.

According to claim 14 of the present invention, a salt core in whichaluminum borate whiskers sufficiently disperse in a salt material madefrom a mixed salt and which is sufficiently reinforced by the whiskerscan be formed. Thus, a rigid salt core having a low melting point can beformed by casting.

According to claim 15 of the present invention, potassium chloride andsodium carbonate are easily available and inexpensive. Thus, accordingto the present invention, the manufacturing cost of a core for use incasting which is made of a salt material made from a mixed salt can bedecreased.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view showing a cylinder block which is castusing a core for use in casting according to the present invention;

FIG. 2 is a graph showing the relationship between the addition ofsynthetic mullite and the bending strength;

FIG. 3 is a graph showing the relationship between the addition ofsynthetic mullite and the bending strength;

FIG. 4 includes views showing a bending sample;

FIG. 5 is a graph showing the relationship between the bending sample;and the bending force;

FIG. 6 is a graph showing the relationship between the addition ofaluminum borate and the bending strength;

FIG. 7 is a graph showing the relationship between the addition ofsilicon nitride and the bending strength;

FIG. 8 is a graph showing the relationship between the addition ofsilicon carbide and the bending strength;

FIG. 9 is a graph showing the relationship between the addition ofaluminum nitride and the bending strength;

FIG. 10 is a graph showing the relationship between the addition ofboron carbide and the bending strength;

FIG. 11 is a graph showing the relationship between the addition ofaluminum titanate or spinel and the bending strength;

FIG. 12 is a graph showing the relationship between the addition ofalumina and the bending strength;

FIG. 13 is a graph showing the relationship between the addition of eachof all the ceramic materials indicated in the first to eighthembodiments and the bending strength:

FIG. 14 is a graph showing the relationship between the addition of eachof all the ceramic materials indicated in the first to eighthembodiments and the bending strength;

FIG. 15 is a chart showing mixing conditions for potassium chloride andthe ceramic material;

FIG. 16 is a chart showing the relationship between the mixing ratio ofthe granular ceramic material and the fluidity;

FIG. 17 is a chart showing the relationship between the mixing ratio ofthe granular ceramic material and the fluidity;

FIG. 18 is a chart showing the relationship between the mixing ratio ofthe granular ceramic material and the fluidity;

FIG. 19 is a graph showing the relationship between the addition ofaluminum borate whiskers and the bending strength;

FIG. 20 is a graph showing the relationship between the addition ofsilicon nitride whiskers or silicon carbide whiskers and the bendingstrength:

FIG. 21 is a graph showing the relationship between the addition ofpotassium titanate whiskers and the bending strength;

FIG. 22 is a graph showing the relationship between the addition of zincoxide whiskers and the bending strength;

FIG. 23 is a graph showing the relationship between the addition of eachof all the whiskers indicated in the ninth to 12th embodiments and thebending strength;

FIG. 24 is a chart showing the relationship between the mixing ratio ofceramic whiskers and the fluidity; and

FIG. 25 is a graph showing the relationship between the addition ofaluminum borate whiskers in potassium bromide or sodium bromide and thebending strength.

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment

A core for use in casting according to one embodiment of the presentinvention will be described in detail with reference to FIGS. 1 to 5.

FIG. 1 is a partially cutaway perspective view of a cylinder block whichis cast using a core for use in casting according to the presentinvention. FIGS. 2 and 3 are graphs each showing the relationshipbetween the addition of synthetic mullite and the banding strength, FIG.4 includes views showing a bending sample, and FIG. 5 is a graph showingthe relationship between the weight of the bending sample and thebending force.

Referring to FIG. 1, reference numeral 1 denotes an engine cylinderblock which is cast using a salt core 2 serving as a core for use incasting according to the present invention. The cylinder block 1 servesto form a motorcycle water-cooling 4-cycle 4-cylinder engine, and isformed into a predetermined shape by die-casting. The cylinder block 1according to this embodiment integrally has a cylinder body 4 havingcylinder bores 3 at four portions and an upper crank case 5 extendingdownward from the lower end of the cylinder body 4. A lower crank case(not shown) is attached to the lower end of the upper crank case 5. Theupper crank case 5 cooperates with the lower crank case to rotatablysupport a crank shaft (not shown).

The cylinder body 4 described above is of a so-called closed deck type,and a water jacket 6 is formed in it using the salt core 2 according tothe present invention. The water jacket 6 comprises a cooling waterinlet 8 which projects from one side of the cylinder body 4 and isformed in a cooling water channel forming portion 7 extending in adirection along which the cylinder bores 3 line up, a cooling waterdistribution channel (not shown) which is formed in the cooling waterchannel forming portion 7, a main cooling water channel 9 whichcommunicates with the cooling water distribution channel and is formedto cover all the cylinder bores 3, a communicating channel 10 whichextends upward in FIG. 1 from the main cooling water channel 9 and opensto a mating surface 4 a at the upper end of the cylinder body 4, and thelike.

More specifically, the water jacket 6 is configured to supply coolingwater, flown into it from the cooling water inlet 8, to the main coolingwater channel 9 around the cylinder bores via the cooling waterdistribution channel and guide the cooling water from the main coolingwater channel 9 to a cooling water channel in a cylinder head (notshown) via the communicating channel 10. As the water jacket 6 is formedin this manner, the cylinder body 4 is covered with the ceiling wall (awall that forms the mating surface 4 a) of the cylinder body 4 exceptthat the communicating channel 10 of the water jacket 6 opens to themating surface 4 a at the upper end of the cylinder body 4 to which thecylinder head is connected, thus forming a closed deck type structure.

The salt core 2 which serves to form the water jacket is formed suchthat it is integrally connected to the respective portions of the waterjacket 6. Referring to FIG. 1, the cylinder body 4 is partially cutawayto facilitate understanding of the shape of the salt core 2 (the shapeof the water jacket 6).

The salt core 2 is formed into the shape of the water jacket 6 bydie-casting using a core material comprising a mixture of a saltmaterial and ceramic material (to be described later). In the salt core2 according to this embodiment, as shown in FIG. 1, a channel formingportion 2 a which forms the cooling water inlet 8 and the cooling waterdistribution channel, an annular portion 2 b which surrounds the fourcylinder bores 3, and a plurality of projections 2 c which projectupward from the annular portion 2 b are all integrally formed. Theprojections 2 c form the communicating channel 10 of the water jacket 6.As is conventionally known, in casting, the salt core 2 is supported ata predetermined position in a mold (not shown) by core prints (notshown). After casting, the salt core 2 is removed by dissolving it withhot water or steam.

To remove the salt core 2 after casting, the cylinder block 1 is dippedin a water tank (not shown) which stores hot water. When the cylinderblock 1 is dipped in the water tank in this manner, the channel formingportion 2 a in the salt core 2 and the projections 2 c exposed to themating surface 4 a are dissolved as they come into contact with the hotwater. The dissolved portion gradually spreads to finally dissolve allthe portions. In the core removing process, hot water or steam may beblown with pressure from a hole to promote dissolution of the salt core2 left in the water jacket 6. In the salt core 2, at portions where theprojections 2 c are to be formed, core prints can be inserted in placeof the projections 2 c.

For example, the salt core 2 according to this embodiment uses syntheticmullite [3Al₂O₃.2SiO₂ {MM-325 mesh manufactured by ITOCHU CERATECHCORP., addition: 40 wt %}] to be described later as the salt material.When forming the salt core 2 by die-casting, first, the mixture of thesalt material and ceramic material is heated to melt the salt material.The melt is stirred such that the ceramic material dispersessufficiently, thus forming a mixed melt. After that, the mixed melt isinjected into a salt core mold with a high pressure and solidified.After the mixed melt solidifies, it is removed from the mold, thusobtaining the salt core 2.

In selection of synthetic mullite as the ceramic material, a pluralityof products shown in Table 1 below were selected from commerciallyavailable granular (powder) synthetic mullite products. Among theselected products, those that could be used for casting were sorted outin accordance with the following experiment. TABLE 1 Particle MaximumChemical Name of Density size Addition in Sample Addition Name ofCeramic Name of Product formulae Shape Manufacturer (g/cm³) (μm) (wt %)(wt %) Synthetic CeraBeads 3Al₂O₃.2SiO₂ = Mullite Particulate ITOCHU2.79  53-106 20, 30, 40, 50, 60, x70 60 mullite/sintered #1700 CERATECHproduct CORP. Synthetic CeraBeads 3Al₂O₃.2SiO₂ = Mullite ParticulateITOCHU 2.79  75-150 40, 50, 60, x70 60 mullite/sintered #1450 CERATECHproduct CORP. Synthetic CeraBeads 3Al₂O₃.2SiO₂ = Mullite ParticulateITOCHU 2.79 106-300 s30, s40, s50, s60, x70 60 mullite/sintered #650CERATECH product CORP. Synthetic MM-325mesh 3Al₂O₃.2SiO₂ = MulliteParticulate ITOCHU 3.11 −45 10, 20, 30, 40, x50 40 mullite/groundCERATECH product CORP. Synthetic MM-200mesh 3Al₂O₃.2SiO₂ = MulliteParticulate ITOCHU 3.11 −75 20, 30, 40 40 mullite/ground CERATECHproduct CORP. Synthetic MM-150mesh 3Al₂O₃.2SiO₂ = Mullite ParticulateITOCHU 3.11 −100 20, 30, 40 40 mullite/ground CERATECH product CORP.Synthetic MM-100mesh 3Al₂O₃.2SiO₂ = Mullite Particulate ITOCHU 3.11 −15020, 30, 40 40 mullite/ground CERATECH product CORP. Synthetic MM35-3Al₂O₃.2SiO₂ = Mullite Particulate ITOCHU 3.11 180-500 s30, s40 40mullite/ground 100mesh CERATECH product CORP. Synthetic MM-16mesh3Al₂O₃.2SiO₂ = Mullite Particulate ITOCHU 3.11 −1000 s20, s30, s40, x5040 mullite/ground CERATECH product CORP. Synthetic MM-325mesh3Al₂O₃.2SiO₂ + 5-10% Particulate ITOCHU 3.15 −45 20, 30, 40 40 mullite +57 Al₂O₃ CERATECH 10% corundum CORP.xNo fluiditysSedimentation

In Table 1, the name of product is an expression which is used by themanufacturer in marketing, and specifies corresponding syntheticmullite. The addition in sample indicates the proportion in weight ofsynthetic mullite added in potassium chloride.

The experiment to sort out from the synthetic mullite products shown inTable 1 those that could be used for casting was performed by heatingthe mixture of potassium chloride and synthetic mullite to dissolvepotassium chloride, stirring the mixture sufficiently, turning thedissolution vessel upside down, and checking the fluidity of the melt inaccordance with whether or not the melt in the vessel flowed out. Bythis experiment, as described above, melts that had fluidity when thedissolution vessel was turned upside down were selected as beingcastable. The result is shown in Table 1 and FIGS. 16 and 17.

As the dissolving vessel described above, a crucible made of INCONELX-750 or a high-alumina Tammann tube was used. Potassium chloride wasdissolved by placing the dissolving vessel containing potassium chloridein an electric resistance furnace and heating it in an atmosphere.Casting was performed by injecting the melt at a temperature of 800° C.into a mold at a temperature of about 25° C. After the casting, in orderto prevent a sample from being fixed to the mold by heat shrinkage, thesample was extracted from the mold at a lapse of about 20 sea since themelt was injected, and was cooled by air cooling at room temperature.

With this experiment, CeraBeads #650 was observed to have fluidity whenits addition was 30%, 40%, 50%, and 60%, as shown in Table 1 and FIG.15. From this result, as CeraBeads #650 sufficiently had fluidity if itsaddition was 60% or less, it was supposedly castable, but could not beused for casting because it sedimented on the bottom of the dissolvingvessel (Table 1 and FIGS. 15 and 16).

CeraBeads #1700 was observed to have fluidity when its addition was 20%,30%, 40%, 50%, and 60%. From this result, CeraBeads #1700 sufficientlyhas fluidity if its addition is 60% or less, and is thus supposed to becastable.

CeraBeads #1450 was observed to have fluidity when its addition was 40%,50%, and 60%. From this result, CoraBeads #1450 sufficiently hasfluidity if its addition is 60% or less, and is thus supposed to becastable. Both CeraBeads #1700 and #1450 were also confirmed to dispersein a melt of potassium chloride (Table 1 and FIGS. 15 and 16).

MM-325 mesh was observed to have fluidity when its addition was 10%,20%, 30%, and 40%. From this result, MM-325 mesh sufficiently hasfluidity if its addition is 40% or less, and is thus supposed to becastable. MM-325 mesh was also confirmed to disperse in a melt ofpotassium chloride (Table 1 and FIGS. 15 and 17).

Each of MM-200 mesh, MM-150 mesh, MM-100 mesh, and SM-325 mesh wasobserved to have fluidity when its addition was 20%, 30%, and 40%. Fromthis result, each of MM-200 mesh, MM-150 mesh, MM-100 mesh, and SM-325mesh has fluidity if its addition is 40% or less, and is thus supposedto be castable. Each of MM-200 mesh, MM-150 mesh, MM-100 mesh, andSM-325 mesh was also confirmed to disperse in a melt of potassiumchloride (Table 1 and FIGS. 15 and 17).

Only MM35 to 100 mesh samples each with an addition of 30% and 50% weresubjected to experiment. With these additions, although fluidity wasobserved, the sample-sedimented on the bottom of the dissolving vessel(see Table 1 and FIG. 15) and was not suitable as the material.

MM-16 mesh samples were observed to have fluidity when its addition was20%, 30%, and 40%, but sedimented on the bottom of the dissolving vesseland were not suitable as the material. In Table 1, CeraBeads is asintered product, and MM is a ground product.

Of these ceramic materials, those that sedimented were excluded exceptMM-16 mesh, and the rest was used. As shown in Tables 2, 3 and 4 below,bending samples were formed for respective additions, and their bendingstrengths were measured. The results shown in FIGS. 2 and 3 wereobtained. TABLE 2 Composition Bending Composition wt % Bending Load NStrength MPa pure KCl 0 186.255 1.55 pure KCl 0 250.024 2.08 pure KCl 0226.274 1.89 pure KCl 0 308.725 2.57 pure KCl 0 225.850 1.88 KCl + 10%MM325 10 588.125 4.90 KCl + 10% MM325 10 770.5 6.42 KCl + 10% MM325 10655.099 5.46 KCl + 10% MM325 10 596.9 4.97 KCl + 10% MM325 10 545.7754.55 KCl + 20% MM325 20 1010 8.42 KCl + 20% MM325 20 923.25 7.69 KCl +20% MM325 20 569.7 4.75 KCl + 20% MM325 20 609.849 5.08 KCl + 20% MM32520 910.325 7.59 KCl + 20% MM325 20 493.925 4.12 KCl + 20% MM325 20 6805.67 KCl + 30% MM325 30 1122.59 9.35 KCl + 30% MM325 30 1263.75 10.53KCl + 30% MM325 30 1060.12 8.83 KCl + 30% MM325 30 1089.57 9.08 KCl +30% MM325 30 716.4 5.97 KCl + 40% MM325 40 1209.5 10.08 KCl + 40% MM32540 1136.25 9.47 KCl + 40% MM325 40 1472.9 12.27 KCl + 40% MM325 40 164213.68 KCl + 40% MM325 40 1584.75 13.21 KCl + 40% MM325 40 1574.8 13.12KCl + 40% MM325 40 1279.75 10.66

TABLE 3 Composition Bending Bending Composition wt % Load N Strength MPapure KCl 0 186.255 1.55 pure KCl 0 250.024 2.08 pure KCl 0 226.274 1.89pure KCl 0 308.725 2.57 pure KCl 0 225.850 1.88 KCl + 20% MM - 200 mesh20 1143.19 9.53 KCl + 30% MM - 200 mesh 30 1083.25 9.03 KCl + 30% MM -200 mesh 30 1216.25 10.14 KCl + 40% MM - 200 mesh 40 1132 9.43 KCl + 40%MM - 200 mesh 40 1740.25 14.50 pure KCl 0 186.255 1.55 pure KCl 0250.024 2.08 pure KCl 0 226.274 1.89 pure KCl 0 308.725 2.57 pure KCl 0225.850 1.88 KCl + 20% MM - 150 mesh 20 922.075 7.68 KCl + 30% MM - 150mesh 30 1119.9 9.33 KCl + 30% MM - 150 mesh 30 1102.84 9.19 KCl + 40%MM - 150 mesh 40 1674.25 13.95 KCl + 40% MM - 150 mesh 40 1822.5 15.19pure KCl 0 186.255 1.55 pure KCl 0 250.024 2.08 pure KCl 0 226.274 1.89pure KCl 0 308.725 2.57 pure KCl 0 225.850 1.88 KCl + 20% MM - 100 mesh20 1072 8.93 KCl + 30% MM - 100 mesh 30 880.5 7.34 KCl + 30% MM - 100mesh 30 1168.57 9.74 KCl + 40% MM - 100 mesh 40 1642.5 13.69 KCl + 40%MM - 100 mesh 40 1579 13.16 pure KCl 0 186.255 1.55 pure KCl 0 250.0242.08 pure KCl 0 226.274 1.89 pure KCl 0 308.725 2.57 pure KCl 0 225.8501.88 KCl + 20% MM - 16 mesh 20 267.875 2.23 KCl + 30% MM - 16 mesh 30364.225 3.04 KCl + 40% MM - 16 mesh 40 485.649 4.05

TABLE 4 Composition Bending Bending Composition wt % Load N Strength MPapure KCl 0 186.255 1.55 pure KCl 0 250.024 2.08 pure KCl 0 226.274 1.89pure KCl 0 308.725 2.57 pure KCl 0 225.850 1.88 KCl + 20% SM - 325 mesh20 1283.75 10.70 KCl + 30% SM - 325 mesh 30 1381.22 11.51 KCl + 30% SM -325 mesh 30 1219.22 10.16 KCl + 40% SM - 325 mesh 40 1708.82 14.24 KCl +40% SM - 325 mesh 40 2029 16.91 pure KCl 0 186.255 1.55 pure KCl 0250.024 2.08 pure KCl 0 226.274 1.89 pure KCl 0 308.725 2.57 pure KCl 0225.850 1.88 KCl + 20% cerabeads#1700 20 802.75 6.69 KCl + 30%cerabeads#1700 30 926 7.72 KCl + 40% cerabeads#1700 40 891.075 7.43KCl + 50% cerabeads#1700 50 1070.02 8.92 KCl + 50% cerabeads#1700 50977.5 8.15 KCl + 60% cerabeads#1700 60 650.75 5.42 KCl + 60%cerabeads#1700 60 915.75 7.63 pure KCl 0 186.255 1.55 pure KCl 0 250.0242.08 pure KCl 0 226.274 1.89 pure KCl 0 308.725 2.57 pure KCl 0 225.8501.88 KCl + 40% cerabeads#1450 40 798.575 6.65 KCl + 50% cerabeads#145050 729.799 6.08 KCl + 50% cerabeads#1450 50 977.75 8.15 KCl + 60%cerabeads#1450 60 739.75 6.16 KCl + 60% cerabeads#1450 60 930.974 7.76pure KCl 0 186.255 1.55 pure KCl 0 250.024 2.08 pure KCl 0 226.274 1.89pure KCl 0 308.725 2.57 pure KCl 0 225.850 1.88 KCl + 30% cerabeads#65030 443.274 3.69 KCl + 40% cerabeads#650 40 379.625 3.16 KCl + 50%cerabeads#650 50 526.599 4.39 KCl + 60% cerabeads#650 60 519.125 4.33KCl + 60% cerabeads#650 60 550.924 4.59

The bending samples of MM-325 mesh were formed 5 pieces for each ofadditions 0% and 10%, 7 pieces for an addition of 20%, 5 pieces for anaddition of 30%, and 8 pieces for an addition of 40%. Each of thebending samples shown in Tables 2, 3, and 4 was formed by casting into arod shape with a width of 18 mm, a height of 20 mm, and a length ofabout 120 mm to have a rectangular section. Each bending sample was castin the same manner as that performed for checking the fluidity describedabove. Namely, potassium chloride and synthetic mullite were placed in acrucible made of INCONEL X-750 or a Tammann tube. The crucible orTammann tube was heated in a furnace to dissolve potassium chloride.After that, the melt was sufficiently stirred and injected into a mold.The temperature of the melt was set to 800° C.

The bending strength was obtained on the basis of a load that broke thebending sample, when the center of the bending sample was supported attwo points spaced apart by 50 mm and the intermediate portion of thesupport points was pressed by a pressing device having two pressingpoints spaced apart by 10 mm, in accordance with the following equation:σ=3 Pm/bh ²  (1)where σ is the bending strength [MPa], P is the bending load [N], m=20mm, b=18 mm, and h=20 mm.

The bending strength of synthetic mullite (MM-325 mesh) increased to besubstantially proportional to the addition, as shown in FIG. 2. Thesolid line in FIG. 2 is an approximate curve drawn by using the methodof least squares. Even when the addition was equal, the bending strengthwas different when a cavity of about 10% was formed in the sample or theaddition of the ceramic material was slightly nonuniform. In order toconfirm this, the bending force of the sample against the weight wasmeasured. The bending force and the weight were substantiallyproportional to each other, as shown in FIG. 5.

Therefore, as is apparent from FIG. 2, the salt core 2 which is obtainedby mixing synthetic mullite (MM-325 mesh) in potassium chloride has amaximum bending strength of about 14 MPa if the addition of syntheticmullite is in the range of 25% to 40%, and has a bending strength (about8 MPa) with which it can be used in die-casting. This fact signifiesthat the salt core 2 according to this embodiment can be used in most ofthe conventional casting methods including die-casting.

As a result, when the salt core 2 is employed, the degrees of freedom incasting, e.g., the pressure during melt injection and the shape of themold, can be increased. The present inventors set the target bendingstrength of a salt core that can also be employed in die-casting to atleast 8 MPa, because the maximum bending strength at the currenttechnological level of a shell core which is said to have a higherstrength than the current salt core is about 6 MPa.

As is apparent from FIG. 3, except MM-16 mesh, CeraBeads #1700,CeraBeads #1450, and CeraBeads #650, ceramic materials made of othersynthetic mullite materials could also obtain high bending strengths inthe same manner as MM-325 mesh.

The salt core 2 could be formed to have a high bending strength in thismanner probably due to the following reason. The density (2.79 g/cm³ to3.15 g/cm³) of synthetic mullite is appropriately higher than thedensity (1.57 g/cm³) of potassium chloride in a molten state. When theindividual grains of synthetic mullite disperse substantially evenly inpotassium chloride in the molten state and solidify, crack progress inthe salt is suppressed. This is apparent from the fact that a sufficientstrength is not obtained with MM-16 mesh or CeraBeads #650 whichsediments.

Potassium chloride as the major component of the salt core 2 isdissolved in hot water, and accordingly the salt core 2 can be removedby dissolving it in hot water after casting. More specifically, when acast product formed by using the salt core 2 is dipped in, e.g., hotwater, the salt core 2 is removed. When compared to a case wherein,e.g., a shell core, is used in the same manner as the conventional saltcore, the cost of the core removing process can be decreased.

The ceramic material mixed in the salt core 2 is only one type ofsynthetic mullite, and separates from potassium chloride when the saltcore 2 is dissolved in water (hot water), as described above. If theseparated ceramic material is collected and dried, it can be recycledeasily. More specifically, since the ceramic material can be recycled,the manufacturing cost of the salt core 2 can be decreased. If aplurality of ceramic materials are used, even when the salt core isdissolved in hot water and recovered, the mixing ratio of the recoveredceramic material becomes unstable and cannot be managed. Thus, theceramic material is difficult to recycle.

Second Embodiment

A salt core according to the present invention can use granular aluminumborate (9Al₂O₃.2B₂O₃) as a ceramic material. When aluminum borate wasmixed in potassium chloride, a bending strength as shown in FIG. 6 wasobtained.

FIG. 6 is a graph showing the relationship between the addition ofaluminum borate and the bending strength. The bending strength shown inFIG. 6 is obtained by conducting the experiment shown in the firstembodiment by using aluminum borate as a ceramic material. The lines inFIG. 6 are approximate curves drawn using the method of least squares.

As aluminum borate to be used for the experiment, three types shown inTable 5 below were selected from commercially available granularproducts. TABLE 5 Particle Maximum Name of Chemical Name of Density sizeAddition in Sample Addition Name of Ceramic Product formulae ShapeManufacturer (g/cm³) (μm) (wt %) (wt %) Aluminum borate Albolite9Al₂O₃.2B₂O₃ Particulate Shikoku 2.93 2.3 10, 15, x20, x30 15 PF03Chemicals Corp. Aluminum borate Albolite 9Al₂O₃.2B₂O₃ ParticulateShikoku 2.93 7.3 10, 15, 20, x30 20 PF08 Chemicals Corp. Aluminum borateAlbolite 9Al₂O₃.2B₂O₃ Particulate Shikoku 2.93 48.92 10, 20, 30, 35, x4035 PC30 Chemicals Corp.xNo fluiditys: Sedimentation

Of the three types of aluminum borate shown in Table 5, judging from thepresence/absence of fluidity, what could be used for casting wereAlbolite PF03 with an addition of 10% and 15%, Albolite PF08 with anaddition of 10%, 15%, and 20%, and Albolite PC30 with an addition of10%, 20%, 30%, and 35% (see Table 5 and FIG. 16). From this result,Albolite PF03 with an addition of 15% or less, Albolite PF08 with anaddition of 20% or less, and Albolite PC30 with an addition of 35% orless sufficiently have fluidity and are supposedly castable.

It was also confirmed that each of these aluminum borate productsdispersed in a melt of potassium chloride (see FIG. 15). These aluminumborate products respectively have densities of 2.93 g/cm³. The particlesizes of Albolite PF03, Albolite PF08, and Albolite PC30 are 2.3 μm, 7.3μm, and 48.92 μm, respectively.

For each of the three types of aluminum borate having different particlesizes described above, bending samples were formed with the respectiveadditions, as shown in Table 6 below, and their bending strengths weremeasured. TABLE 6 Composition Bending Bending Composition wt % Load NStrength MPa pure KCl 0 186.255 1.55 pure KCl 0 250.024 2.08 pure KCl 0226.274 1.89 pure KCl 0 308.725 2.57 pure KCl 0 225.850 1.88 KCl + 10%Albolite PF03 10 986.250 8.22 KCl + 10% Albolite PF03 10 984.750 8.21KCl + 10% Albolite PF03 10 1027.250 8.56 KCl + 10% Albolite PF03 101298.420 10.82 KCl + 10% Albolite PF03 10 981.000 8.18 KCl + 10%Albolite PF03 10 972.375 8.10 KCl + 10% Albolite PF03 10 1033.000 8.61KCl + 10% Albolite PF03 10 1046.370 8.72 KCl + 15% Albolite PF03 151343.84 11.20 KCl + 15% Albolite PF03 15 1187 9.89 pure KCl 0 186.2551.55 pure KCl 0 250.024 2.08 pure KCl 0 226.274 1.89 pure KCl 0 308.7252.57 pure KCl 0 225.850 1.88 KCl + 10% Albolite PF08 10 897.000 7.48KCl + 10% Albolite PF08 10 1173.070 9.78 KCl + 10% Albolite PF08 101017.250 8.48 KCl + 10% Albolite PF08 10 1138.000 9.48 KCl + 10%Albolite PF08 10 991.275 8.26 KCl + 10% Albolite PF08 10 1199.750 10.00KCl + 10% Albolite PF08 10 1032.090 8.60 KCl + 15% Albolite PF08 151075.500 8.96 KCl + 20% Albolite PF08 20 1145.020 9.54 KCl + 20%Albolite PF08 20 1210.270 10.09 pure KCl 0 186.255 1.55 pure KCl 0250.024 2.08 pure KCl 0 226.274 1.89 pure KCl 0 308.725 2.57 pure KCl 0225.850 1.88 KCl + 10% Albolite PC30 10 793.474 6.61 KCl + 20% AlbolitePC30 20 1126.25 9.39 KCl + 20% Albolite PC30 20 1320.4 11.00 KCl + 30%Albolite PC30 30 1541.75 12.85 KCl + 30% Albolite PC30 30 1415.72 11.80KCl + 35% Albolite PC30 35 1787.55 14.90

When aluminum borate was to be used as a ceramic material in thismanner, as shown in FIG. 6, if the addition was 10% to 20%, the bendingstrength became higher than 8 MPa.

As shown in FIG. 6, the bending strength of aluminum borate is rarelyadversely affected by the particle size.

Therefore, when aluminum borate is used as a ceramic material, asdescribed above, the same effect as that obtained when the firstembodiment is employed can be obtained.

Third Embodiment

A salt core according to the present invention can use granular siliconnitride (Si₃N₄) as a ceramic material. When silicon nitride was mixed inpotassium chloride, a bending strength as shown in FIG. 7 was obtained.

FIG. 7 is a graph showing the relationship between the addition ofsilicon nitride and the bending strength. The bending strength shown inFIG. 7 is obtained by conducting the experiment shown in the firstembodiment by using silicon nitride as a ceramic material. The lines inFIG. 7 are approximate curves drawn using the method of least squares.

As silicon nitride to be used for the experiment, four types shown inTable 7 below were, selected from commercially available granularproducts. TABLE 7 Particle Maximum Name of Name of Chemical Name ofDensity size Addition in Sample Addition Ceramic Product formulae ShapeManufacturer (g/cm³) (μm) (wt %) (wt %) Silicon NP-600 Si₃N₄ ParticulateDENKI KAGAKU 3.18 0.7 20, 24, 33, 25, x30, x35, x40 25 nitride KOGYOK.K. Silicon MM-5MF Si₃N₄ Particulate YAKUSHIMA 3.19 0.8 10, 20, 25, x3025 nitride DENKO CO., LTD. Silicon SN-7 Si₃N₄ Particulate DENKI KAGAKU3.18 4.3 20, 30, 40, x45 40 nitride KOGYO K.K. Silicon SN-9 Si₃N₄Particulate DENKI KAGAKU 3.18 5.7 20, 30, 35, 40 40 nitride KOGYO K.K.xNo fluiditys: Sedimentation

Of the four types of aluminum borate shown in Table 7, judging from thepresence/absence of fluidity, what could be used for casting were NP-600with an addition of 20% and 25%, SN-7 with an addition of 20%, 30%, and40%, SN-9 with an addition of 20%, 30%, 35%, and 40%, and HM-5MF with anaddition of 10%, 20%, and 25%. From this result, NP-600 with an additionof 25% or less, SN-7 with an addition of 40% or less, SN-9 product withan addition of 40% or less, and HM-5 MF with an addition of 25% or lessare supposedly castable.

It was also confirmed that each of the four ceramic materials dispersedin a melt of potassium chloride (See FIG. 15).

NP-600, SN-7, and SN-9 respectively have densities of 3.18 g/cm³, andHM-5MF has a density of 3.19 g/cm³. The four types of silicon nitrideproducts have different particle sizes.

For each of the four types of silicon nitride described above, bendingsamples were formed with the respective additions, as shown in Table 8below, and their bending strengths were measured. TABLE 8 CompositionBending Bending Composition wt % Load N Strength MPa pure KCl 0 186.2551.55 pure KCl 0 250.024 2.08 pure KCl 0 226.274 1.89 pure KCl 0 308.7252.57 pure KCl 0 225.850 1.88 KCl + 20% Si₃N₄ SN-9 20 1056.57 8.80 KCl +20% Si₃N₄ SN-9 20 997.325 8.31 KCl + 30% Si₃N₄ SN-9 30 1163.92 9.70KCl + 30% Si₃N₄ SN-9 30 1038.25 8.65 KCl + 35% Si₃N₄ SN-9 35 1084.3 9.04KCl + 40% Si₃N₄ SN-9 40 1470.5 12.25 pure KCl 0 186.255 1.55 pure KCl 0250.024 2.08 pure KCl 0 226.274 1.89 pure KCl 0 308.725 2.57 pure KCl 0225.850 1.88 KCl + 20% Si₃N₄ SN-7 20 1242.62 10.36 KCl + 20% Si₃N₄ SN-720 948.25 7.90 KCl + 20% Si₃N₄ SN-7 20 1254 10.45 KCl + 30% Si₃N₄ SN-730 1048.84 8.74 KCl + 40% Si₃N₄ SN-7 40 995 8.29 KCl + 40% Si₃N₄ SN-7 401144.25 9.54 pure KCl 0 186.255 1.55 pure KCl 0 250.024 2.08 pure KCl 0226.274 1.89 pure KCl 0 308.725 2.57 pure KCl 0 225.850 1.88 KCl + 20%Si₃N₄ NP-600 20 787.75 6.56 KCl + 20% Si₃N₄ NP-600 20 712.424 5.94 KCl +24.33% Si₃N₄ NP-600 24.33 833.174 6.94 KCl + 25% Si₃N₄ NP-600 25 10308.58 pure KCl 0 186.255 1.55 pure KCl 0 250.024 2.08 pure KCl 0 226.2741.89 pure KCl 0 308.725 2.57 pure KCl 0 225.850 1.88 KCl + 10% Si₃N₄HM-5MF 10 624.849 5.21 KCl + 20% Si₃N₄ HM-5MF 20 917.299 7.64 KCl + 20%Si₃N₄ HM-5MF 20 914.224 7.62 KCl + 25% Si₃N₄ HM-5MF 25 992.9 8.27 KCl +25% Si₃N₄ HM-5MF 25 1134.8 9.46

When silicon nitride was to be used as a ceramic material in thismanner, as shown in FIG. 7, if the addition was 20% or more, the bendingstrength became higher than 8 MPa.

As shown in FIG. 7, the bending strength of silicon nitride is rarelyadversely affected by the particle size.

Therefore, when silicon nitride is used as a ceramic material, asdescribed above, the same effect as that obtained when the firstembodiment is employed can be obtained.

Fourth Embodiment

A salt core according to the present invention can use granular siliconcarbide (SiC) as a ceramic material. When silicon carbide was mixed inpotassium chloride, a bending strength as shown in FIG. 8 was obtained.

FIG. 8 is a graph showing the relationship between the addition ofsilicon carbide and the bending strength. The bending strength shown inFIG. 8 is obtained by conducting the experiment shown in the firstembodiment by using silicon carbide as a ceramic material. The lines inFIG. 8 are approximate curves drawn using the method of least squares.

As silicon carbide to be used for the experiment, three types shown inTable 9 below were selected from commercially available granularproducts. TABLE 9 Particle Maximum Name of Name of Chemical Name ofDensity size Addition in Sample Addition Ceramic Product formulae ShapeManufacturer (g/cm³) (μm) (wt %) (wt %) Silicon OY-15 SiC ParticulateYAKUSHIMA 3.23 0.7 10, 20, 30, 40, 45 45 carbide DENKO CO., LTD. SiliconOY-7 SiC Particulate YAKUSHIMA 3.23 2 10, 20, 30, 40, 45 45 carbideDENKO CO., LTD. Silicon OY-3 SiC Particulate YAKUSHIMA 3.23 3 10, 20,30, 40, 45 45 carbide DENKO CO., LTD.x: No fluiditys: Sedimentation

Of the three types of silicon carbide shown in Table 9, judging from thefluidity, those with additions of 10%, 20%, 30%, 40%, and 45% could beused for casting (see FIG. 18). From this result, any one of the threetypes of silicon carbide is supposedly castable if the addition is 45%or less.

It was also confirmed that each of these silicon carbide productsdispersed in a melt of potassium chloride (see FIG. 15). These siliconcarbide products respectively have densities of 3.23 g/cm³ but differentparticle sizes.

For each of the three types of silicon carbide described above, bendingsamples were formed with the respective additions, as shown in Table 10below, and their bending strengths were measured. TABLE 10 CompositionBending Bending Composition wt % Load N Strength MPa pure KCl 0 186.2551.55 pure KCl 0 250.024 2.08 pure KCl 0 226.274 1.89 pure KCl 0 308.7252.57 pure KCl 0 225.850 1.88 KCl + 20% SiC OY-3 20 964 8.03 KCl + 30%SiC OY-3 30 912.25 7.60 KCl + 30% SiC OY-3 30 1134.75 9.46 KCl + 45% SiCOY-3 45 1263.75 10.53 pure KCl 0 186.255 1.55 pure KCl 0 250.024 2.08pure KCl 0 226.274 1.89 pure KCl 0 308.725 2.57 pure KCl 0 225.850 1.88KCl + 20% SiC OY-7 20 952.75 7.94 KCl + 30% SiC OY-7 30 1292.5 10.77KCl + 30% SiC OY-7 30 954.95 7.96 KCl + 40% SiC OY-7 40 1206.75 10.06KCl + 45% SiC OY-7 45 1185.69 9.88 pure KCl 0 186.255 1.55 pure KCl 0250.024 2.08 pure KCl 0 226.274 1.89 pure KCl 0 308.725 2.57 pure KCl 0225.850 1.88 KCl + 20% SiC OY-15 20 669.75 5.58 KCl + 30% SiC OY-15 30799 6.66 KCl + 30% SiC OY-15 30 673 5.61 KCl + 40% SiC OY-15 40 911.5997.60 KCl + 45% SiC OY-15 45 991.5 8.26

When silicon carbide was to be used as a ceramic material in thismanner, as shown in FIG. 8, if the addition was 25% to 30% or more, thebending strength became higher than 8 MPa.

As shown in FIG. 8, the bending strength of silicon carbide is rarelyadversely affected by the particle size.

Therefore, when silicon carbide is used as a ceramic material, asdescribed above, the same effect as that obtained when the firstembodiment is employed can be obtained.

Fifth Embodiment

A salt core according to the present invention can use granular aluminumnitride (AlN) as a ceramic material. When aluminum nitride was mixed inpotassium chloride, a bending strength as shown in FIG. 9 was obtained.

FIG. 9 is a graph showing the relationship between the addition ofaluminum nitride and the bending strength. The bending strength shown inFIG. 9 is obtained by conducting the experiment shown in the firstembodiment by using aluminum nitride as a ceramic material. The lines inFIG. 9 are approximate curves drawn using the method of least squares.

As aluminum nitride to be used for the experiment, two types shown inTable 11 below were selected from commercially available granularproducts. TABLE 11 Particle Maximum Name of Name of Chemical Name ofDensity size Addition in Sample Addition Ceramic Product formulae ShapeManufacturer (g/cm³) (μm) (wt %) (wt %) Aluminum −250mesh AlNParticulate K.K. TACHYON 3.25 −60 20, 30, 40 40 nitride Aluminum−150mesh AlN Particulate K.K. TACHYON 3.25 −100 20, 30, 40 40 nitridex: No fluiditys: Sedimentation

Of the two types of silicon carbide shown in Table 11, judging from thefluidity, those with additions of 20%, 30%, and 40% could be used forcasting (see Table 11 and FIG. 18). From this result, both of the twotypes of aluminum nitride are supposedly castable if the additions are40%.

It was also confirmed that each of these aluminum nitride productsdispersed in a melt of potassium chloride (see FIG. 15). These aluminumnitride products respectively have densities of 3.25 g/cm³ but differentparticle sizes.

For each of the two types of aluminum nitride described above, bendingsamples were formed with the respective additions, as shown in Table 12below, and their bending strengths were measured. TABLE 12 CompositionBending Bending Composition wt % Load N Strength MPa pure KCl 0 186.2551.55 pure KCl 0 250.024 2.08 pure KCl 0 226.274 1.89 pure KCl 0 308.7252.57 pure KCl 0 225.850 1.88 KCl + 20% AlN - 150 mesh 20 1237.5 10.31KCl + 30% AlN - 150 mesh 30 1503 12.53 KCl + 30% AlN - 150 mesh 301649.5 13.75 KCl + 40% AlN - 150 mesh 40 1730.72 14.42 KCl + 40% AlN -150 mesh 40 2232.25 18.60 pure KCl 0 186.255 1.55 pure KCl 0 250.0242.08 pure KCl 0 226.274 1.89 pure KCl 0 308.725 2.57 pure KCl 0 225.8501.88 KCl + 20% AlN - 250 mesh 20 1422.75 11.86 KCl + 30% AlN - 250 mesh30 1848.75 15.41 KCl + 30% AlN - 250 mesh 30 1922.75 16.02 KCl + 40%AlN - 250 mesh 40 2775.5 23.13 KCl + 40% AlN - 250 mesh 40 2092.89 17.44

When aluminum nitride was to be used as a ceramic material in thismanner, as shown in FIG. 9, if the addition wan 15% or more, the bendingstrength became higher than 8 MPa.

As shown in FIG. 9, the bending strength of aluminum nitride is rarelyadversely affected by the particle size.

Therefore, when aluminum nitride is used as a ceramic material, asdescribed above, the same effect as that obtained when the firstembodiment is employed can be obtained.

Sixth Embodiment

A salt core according to the present intention can use granular boroncarbide (B₄C) as a ceramic material. When boron carbide was mixed inpotassium chloride, a bending strength as shown in FIG. 10 was obtained.

FIG. 10 is a graph showing the relationship between the addition ofboron carbide and the bending strength. The bending strength shown inFIG. 10 is obtained by conducting the experiment shown in the firstembodiment by using boron carbide as a ceramic material. The lines inFIG. 10 are approximate curves drawn using the method of least squares.

As boron carbide to be used for the experiment, three types shown inTable 13 below were selected from commercially available granularproducts. TABLE 13 Particle Maximum Name of Name of Chemical Name ofDensity size Addition in Sample Addition Ceramic Product Formulae ShapeManufacturer (g/cm³) (μm) (wt %) (wt %) Boron carbide #I200 B₄CParticulate DENKI KAGAKU 2.51 −3 20, 30, 33, 75, x35, x40 33.75 KOGYOK.K. Boron carbide S1 B₄C Particulate DENKI KAGAKU 2.51 45-90 20, 30, 4040   KOGYO K.K. Boron carbide S3 B₄C Particulate DENKI KAGAKU 2.51125-250 s20, s30, s40 above 40 KOGYO K.K.xNo fluiditysSedimentation

Of the three types of boron carbide shown in Table 13, judging from thefluidity, what could be used for casting were #1200 with an addition of20%, 30% and 33.75% and S1 and S3 each with an addition of 20%, 30%, and40% (see Table 13 and FIG. 16). From this result, #1200 is supposedlycastable if the addition is 33.75% or less, and S1 and S3 are supposedlycastable if the additions are 40% or less. It was also confirmed that ofeach of the three types of boron carbide, S3 sedimented in a melt ofpotassium chloride while each of the remaining #1200 and S1 dispersed(see FIG. 15). These boron carbide samples respectively have densitiesof 2.15 g/cm³ but different granular sizes.

For each of the three types of boron carbide described above, bendingsamples were formed with the respective additions, as shown in Table 14below, and their bending strengths were measured. TABLE 14 CompositionBending Bending Composition wt % Load N strength Mpa pure KCl 0 186.2551.55 pure KCl 0 250.024 2.08 pure KCl 0 226.274 1.89 pure KCl 0 308.7252.57 pure KCl 0 225.850 1.88 KCl + 20% B₄C #1200 20 1260.84 10.51 KCl +30% B₄C #1200 30 1033 8.61 KCl + 30% B₄C #1200 30 1579 13.16 KCl +33.75% B₄C #1200 33.75 2008 16.73 pure KCl 0 186.255 1.55 pure KCl 0250.024 2.08 pure KCl 0 226.274 1.89 pure KCl 0 308.725 2.57 pure KCl 0225.850 1.88 KCl + 20% B₄C S1 20 924.424 7.70 KCl + 30% B₄C S1 301091.57 9.10 KCl + 30% B₄C S1 30 1281.5 10.68 KCl + 40% B₄C S1 401627.19 13.56 KCl + 40% B₄C S1 40 1265 10.54 pure KCl 0 186.255 1.55pure KCl 0 250.024 2.08 pure KCl 0 226.274 1.89 pure KCl 0 308.725 2.57pure KCl 0 225.850 1.88 KCl + 20% B₄C S3 20 352.149 2.93 KCl + 30% B₄CS3 30 474 3.95 KCl + 30% B₄C S3 30 482.424 4.02 KCl + 40% B₄C S3 40473.125 3.94

When boron carbide was to be used as a ceramic material in this manner,as shown in FIG. 10, if the addition was set to 20% or more in thesample with a sample name #1200 and the sample with a sample name S1,the bending strength became higher than 8 MPa. As shown in FIG. 10, withS3 which disperses, a high strength cannot be obtained.

Therefore, when boron carbide is used as a ceramic material, asdescribed above, the same effect as that obtained when the firstembodiment is employed can be obtained.

Seventh Embodiment

A salt core according to the present invention can use granular aluminumtitanate (Al₂TiO₅) or spinal (cordierite: MgO.Al₃O₃) as a ceramicmaterial. When such a ceramic material was mixed in potassium chloride,a bending strength as shown in FIG. 11 was obtained.

FIG. 11 is a graph showing the relationship between the addition ofaluminum titanate or spinal and the bending strength. The bendingstrength-shown in FIG. 11 is obtained by conducting the experiment shownin the first embodiment by using aluminum titanate or spinel as aceramic material. The lines in FIG. 11 are approximate curves drawnusing the method of least squares.

As aluminum titanate and spinal to be used for the experiment, thoseshown in Table 15 below were selected from commercially availablegranular products. TABLE 15 Particle Maximum Name of Name of ChemicalName of Density size Addition in Sample Addition Ceramic Productformulae Shape Manufacturer (g/cm³) (μm) (wt %) (wt %) Spinel NSP-70-MgO.Al₂O₃ Particulate ITOCHU 3.27 75 20, 30, 40, x50 40 200mesh CERATECHCORP. Aluminum VCAT Al₂TiO₅ Particulate Shinku 3.7- −1.0 10, 20, 30, 40,x50 40 titanate Ceramics K.K.xNo fluiditys: Sedimentation

Of aluminum titanate shown in Table 13, judging from the fluidity, thosewith additions of 10%, 20%, 30% and 40% could be used for casting, andof spinel, judging from the fluidity, those with additions of 20%, 30%,and 40% could be used for casting (see Table 15 and FIG. 18). From thisresult, aluminum titanate and spinel are supposedly castable if theadditions are 40% or less. It was also confirmed that each of the twoceramic materials dispersed in a melt of potassium chloride (see FIG.15).

Aluminum titanate has a density of 3.7 g/cm³ and a particle size of 1μm, and spinel has a density of 3.27 g/cm³ and a particle size of 75 μm.

For each of the ceramic materials described above, bending samples wereformed with the respective additions, as shown in Table 16 below, andtheir bending strengths were measured. TABLE 16 Composition BendingBending Composition wt % Load N Strength MPa pure KCl 0 186.255 1.55pure KCl 0 250.024 2.08 pure KCl 0 226.274 1.89 pure KCl 0 308.725 2.57pure KCl 0 225.850 1.88 KCl + 20% Al₂TiO₅ 20 749.25 6.24 KCl + 30%Al₂TiO₅ 30 1336.55 11.14 KCl + 30% Al₂TiO₅ 30 1270.07 10.58 KCl + 40%Al₂TiO₅ 40 1137.19 9.48 KCl + 40% Al₂TiO₅ 40 1341.75 11.18 pure KCl 0186.255 1.55 pure KCl 0 250.024 2.08 pure KCl 0 226.274 1.89 pure KCl 0308.725 2.57 pure KCl 0 225.850 1.88 KCl + 20% MgO.Al₂O₃ 20 1111.07 9.26KCl + 30% MgO.Al₂O₃ 30 1541.87 12.85 KCl + 30% MgO.Al₂O₃ 30 1453 12.11KCl + 40% MgO.Al₂O₃ 40 1892.75 15.77 KCl + 40% MgO.Al₂O₃ 40 1898.7515.82

When aluminum titanate or spinel was to be used as a ceramic material inthis manner, as shown in FIG. 11, if the addition was set to 20% ormore, the bending strength became higher than 8 MPa, as shown in FIG.11.

Therefore, when aluminum titanate or spinel is used as a ceramicmaterial, as described above, the same effect as that obtained when thefirst embodiment is employed can be obtained.

Eighth Embodiment

A salt core according to the present invention can use granular alumina(Al₂O₃) as a ceramic material. When such alumina was mixed in potassiumchloride, a bending strength as shown in FIG. 12 was obtained.

FIG. 12 is a graph showing the relationship between the addition ofalumina and the bending strength. The bending strength shown in FIG. 12is obtained by conducting the experiment shown in the first embodimentby using alumina as a ceramic material. The lines in FIG. 12 areapproximate curves drawn using the method of least squares.

As alumina to be used for the experiment, those shown in Table 17 belowwere selected from commercially available granular products. TABLE 17Particle Maximum Name of Name of Chemical Name of Density size Additionin Sample Addition Ceramic Product formulae Shape Manufacturer (g/cm³)(μm) (wt %) (wt %) Alumina AL-160SG-3 Al₂O₃ Particulate SHOWA DENKO K.K.3.92 0.6 20, 30, x35, x40 30 Alumina AL-45-1 Al₂O₃ Paritculate SHOWADENKO K.K. 3.93 1   20, 30, 35, x40 35 Alumina A-42-1 Al₂O₃ ParticulateSHOWA DENKO K.K. 3.95 3-4 20, 30, x35, x40 30 Alumina A-12 Al₂O₃Particulate SHOWA DENKO K.K. 3.96 40-50 20, 30, x35 30xNo fluiditys: Sedimentation

Of alumina samples shown in Table 17, judging from the fluidity, thosewith additions of 20%, 20%, 30% and 35% (AL-45-1) could be used forcasting (see FIG. 18). From this result, AL-45-1 is supposedly castableif the addition is 35% or less, and the remaining samples are supposedlycastable if the additions are 30% or less.

It was also confirmed that any one of the above alumina samplesdispersed in a melt of potassium chloride (see FIG. 15). These aluminasamples have densities of about 4 g/cm³ and particle sizes of 0.6 μm(AL-160SG), 1 μm (AL-45-1), 3 μm to 4 μm (A-42-1), and 40 μm to 50 μm(A-12).

For each of alumina samples described above, bending samples were formedwith the respective additions, as shown in Table 18 below, and theirbending strengths were measured. TABLE 18 Composition Bending BendingComposition wt % Load N Strength MPa pure KCl 0 186.255 1.55 pure KCl 0250.024 2.08 pure KCl 0 226.274 1.89 pure KCl 0 308.725 2.57 pure KCl 0225.850 1.88 KCl + 20% Al₂O₃ AL-45-1 20 1041.25 8.68 KCl + 30% Al₂O₃AL-45-1 30 1037.05 8.64 KCl + 35% Al₂O₃ AL-45-1 35 1116 9.30 KCl + 35%Al₂O₃ AL-45-1 35 1008.67 8.41 pure KCl 0 186.255 1.55 pure KCl 0 250.0242.08 pure KCl 0 226.274 1.89 pure KCl 0 308.725 2.57 pure KCl 0 225.8501.88 KCl + 20% Al₂O₃ A-42-1 20 871.75 7.26 KCl + 20% Al₂O₃ A-42-1 201432.5 11.94 KCl + 30% Al₂O₃ A-42-1 30 2118.07 17.65 KCl + 30% Al₂O₃A-42-1 30 1660.75 13.84 pure KCl 0 186.255 1.55 pure KCl 0 250.024 2.08pure KCl 0 226.274 1.89 pure KCl 0 308.725 2.57 pure KCl 0 225.850 1.88KCl + 20% Al₂O₃ A-12 20 1093.52 9.11 KCl + 20% Al₂O₃ A-12 20 972.4 8.10KCl + 30% Al₂O₃ A-12 30 1456 12.13 KCl + 30% Al₂O₃ A-12 30 1540 12.83pure KCl 0 186.255 1.55 pure KCl 0 250.024 2.08 pure KCl 0 226.274 1.89pure KCl 0 308.725 2.57 pure KCl 0 225.850 1.88 KCl + 20% Al₂O₃ 20973.75 8.11 AL-160SG-3 KCl + 20% Al₂O₃ 20 986.25 8.22 AL-160SG-3 KCl +30% Al₂O₃ 30 1166.34 9.72 AL-160SG-3 KCl + 30% Al₂O₃ 30 1183.75 9.86AL-160SG-3

When alumina was to be used as a ceramic material in this manner, asshown in FIG. 12, if the addition was set to 20% or more, the bendingstrength became higher than 8 MPa.

Therefore, when alumina is used as a ceramic material, as describedabove, the same effect as that obtained when the first embodiment isemployed can be obtained.

FIGS. 13 and 14 show the relationship between the additions of all theceramic materials indicated in the first to eighth embodiments describedabove and the bending strengths. As is apparent from FIGS. 13 and 14, ofthe ceramic materials described above, what could form a salt core withthe highest bending strength was aluminum nitride.

Of the ceramic materials described above, the one with the leastexpensive material unit cost is synthetic mullite, and the one thatrequires the minimum material amount (addition) is aluminum borate. Morespecifically, when synthetic mullite or aluminum borate is used, a saltcore having a high strength can be manufactured while suppressing themanufacturing cost.

When the ceramic material indicated in any one of the first to eighthembodiments was used, a salt core with excellent castability and highstrength could be formed probably because of the following reason. Amelt obtained by mixing such a ceramic material in potassium chloridehas fluidity. The density of the ceramic material is appropriatelyhigher than the density (1.57 g/cm³) of potassium chloride in a moltenstate. Such a ceramic material disperses in potassium chloride in themolten state widely and evenly to suppress crack progress in the salt.

More specifically, “fluidity” enabled casting, and “dispersion” enabledsufficient strength. Of the two factors, “fluidity” is influenced mainlyby the addition (wt %) of the ceramic material, and “dispersion” isinfluenced by the density. Even a ceramic material different from thosedescribed in the first to eighth embodiments is supposedly able to forma salt core having the equal strength to those indicated in theembodiments described above, as far as the different ceramic materialhas a density approximate to those of the ceramic materials describedabove so that it forms a melt having fluidity.

In order to investigate whether the ceramic material disperses well inthe salt material in the molten state, the present inventors conductedan experiment on the mixing conditions of potassium chloride and theceramic material. According to this experiment, as shown in FIG. 15, aceramic material which dispersed in molten potassium chloride had aminimum density which is higher than 2.28 g.cm³ (boron nitride), amaximum density of 4 g/cm³ (alumina), and a maximum particle size ofabout 150 μm.

This is because dispersion is closely related to the solidification timeof the melt and the sedimentation velocity of the ceramic material. Thetheoretical equation of the sedimentation velocity is:V=g(ρc−ρs)d ²/18μ  (2)where V is the sedimentation velocity [m/s], g is the gravitationalacceleration 9.80 [m/s²], ρc is the density [g/cm³] of the ceramicmaterial. ρs is the density [g/cm³] of the salt material in the moltenstate, d is the particle size [m] of the ceramic material, and μ is thecoefficient of viscosity [Pa·s] of the salt material.

According to equation (2), the sedimentation velocity V is proportionalto the density difference between the ceramic material and the saltmaterial in the molten state and to the square of the particle size.Hence, regarding the particle size, if it is larger than 150 μm, thesedimentation velocity becomes very fast so the ceramic material may notbe able to be dispersed well. Regarding the density of the ceramicmaterial, it influences the sedimentation velocity more than theparticle size does. Thus, even a ceramic material having a densityhigher than 4 g/cm³, which is not subjected to the experiment this time,can be estimated to be dispersed well.

The relationship between the additions of the respective ceramicmaterials and the fluidities were as shown in FIGS. 16 to 18. Theresults of FIGS. 16 to 18 were obtained by an experiment of placing theceramic material and potassium chloride in a Tammann tube, dissolvingthe mixture at 800° C., stirring the mixture sufficiently, and reversingthe Tammann tube upside down. Of the mixtures, one the melt of whichflowed out from the Tammann tube was determined as “with fluidity” andone the melt of which did not was determined as “without fluidity”.

Therefore, any ceramic material that has a density falling within arange of 2.2 g/cm³ (=the density of boron nitride) (exclusive) to 4g/cm³ (inclusive) or/and a particle size of about 150 μm or less, formsgrains, and disperses in a melt of potassium chloride sufficiently canform a salt core having such a strength that it can be used indie-casting as well.

Ninth Embodiment

A salt core according to the present invention can use aluminum boratewhiskers (9Al₂O₃.2B₂O₃), silicon nitride whiskers (Si₃N₄), siliconcarbide whiskers (SiC), potassium hexatitanate whiskers (K₂O.6TiO₂),potassium octatitanate whiskers (K₂O.8TiO₂), or zinc oxide whiskers(ZnO) as a ceramic material. Examples of the ceramic whiskers includethose shown in Table 19 below. TABLE 19 Particle Maximum Name of Name ofChemical Name of Density size Particle Addition in Sample AdditionCeramic Product formulae Shape Manufacturer (g/cm³) (μm) size (μm) (wt%) (wt %) Aluminum Albolex 9Al₂O₃.2B₂O₃ whisker Shikoku 2.93 10-300.5-1.0 10, 15, 18.67, x20 15 borate M20 Chemicals Corp. Silicon SNW#1-S Si₃N₄ alpha whisker Tateho 3.18  5-200 0.1-1.6 5, 7, x8 7 nitrideChemical Industries Co., Ltd. Silicon SCW SiC Beta whisker Tateho 3.18 5-200 0.05-1.5  5, 7, x8, x10, x15 7 carbide #1-0.8 Chemical IndustriesCo., Ltd. Potassium Tismo N H₂O.6TiO₂ whisker Otsuka (3.4-3.6) 10-200.3-0.6 5, 7, x8, xI0 7 hexatitanate Chemical 3.58 Co., Ltd. PotassiumTismo D K₂O.8TiO₂ whisker Otsuka (3.4-3.6) 10-20 0.3-0.6 5, 7, x8, x10 7octatitanate Chemical 3.58 Co., Ltd. Zinc oxide WZ-0501 ZnO whiskerMatsushita 5.78  2-50 0.2-3.0 5, 10, 15, x16, 15 AMTEC K.K. x18, x20xNo fluiditys: Sedimentation

As shown in Table 19, of aluminum borate whiskers (tradename: AlbolexM20), judging from the fluidity, those with additions of 10%, 15, and18.67% could be used for casting (see FIG. 24). From this result,aluminum borate whiskers are supposedly castable if the addition is18.67% or less.

Of silicon nitride whiskers (tradename: SNW #1-S), silicon carbidewhiskers (tradename: SCW #1-0.8), potassium hexatitanate whiskers(tradename: Tismo N), and potassium octatitanate whiskers (tradename:Tismo D), those with additions of 5% and 7% could be used for casting(see FIG. 24). From this result, these whiskers are supposedly castableif the addition is 7% or less.

Of zinc oxide whiskers (tradename: WZ-0501), those with additions of 5%,10%, and 15% could be used for casting (see FIG. 24). From this result,zinc oxide whiskers are supposedly castable if the addition is 15% orless.

Of these whiskers, when aluminum borate whiskers were mixed in potassiumchloride, a bending strength as shown in FIG. 19 was obtained.

FIG. 19 is a graph showing the relationship between the addition ofaluminum borate whiskers and the bending strength. The bending strengthshown in FIG. 19 is obtained by conducting the experiment shown in thefirst embodiment by using aluminum borate whiskers as a ceramicmaterial. The line in FIG. 19 is an approximate curve drawn using themethod of least squares. When conducting this experiment, bendingsamples were formed with the respective additions, as shown in Table 20below, and their bending strengths were measured. TABLE 20 BendingComposition Bending Strength Composition wt % Load N MPa pure KCl 0186.255 1.55 pure KCl 0 250.024 2.08 pure KCl 0 226.274 1.89 pure KCl 0308.725 2.57 pure KCl 0 225.850 1.88 KCl + 10% Albolex M20 10 2485.75020.71 KCl + 10% Albolex M20 10 2466.75 20.56 KCl + 10% Albolex M20 102488.75 20.74 KCl + 10% Albolex M20 10 2832.25 23.60 KCl + 10% AlbolexM20 10 2262.89 18.86 KCl + 10% Albolex M20 10 2758.00 22.98 KCl + 10%Albolex M20 10 2624.75 21.87 KCl + 10% Albolex M20 10 2155.35 17.96KCl + 15% Albolex M20 15 4101.05 34.18 KCl + 15% Albolex M20 15 3722.7531.02 KCl + 15% Albolex M20 15 3763.50 31.36 KCl + 15% Albolex M20 153973.75 33.11 KCl + 15% Albolex M20 15 3305.72 27.55 KCl + 15% AlbolexM20 15 3783.02 31.53 KCl + 15% Albolex M20 15 3411.75 28.43 KCl + 18.7%Albolex M20 18.7 4346.25 36.22

When aluminum borate whiskers were to be used as a ceramic material inthis manner, as shown in FIG. 19, if the addition was 5% or more, thebending strength became higher than 8 MPa. If the addition was 18% ormore, a bending strength of as high as 35 MPa was exhibited.

Therefore, when aluminum borate whiskers are used as a ceramic material,as described above, the same effect as that obtained when the firstembodiment is employed can be obtained.

10th Embodiment

When silicon nitride whiskers or silicon carbide whiskers were mixed inpotassium chloride, a bending strength as shown in FIG. 20 was obtained.

FIG. 20 is a graph showing the relationship between the addition ofsilicon nitride whiskers or the addition of silicon carbide whiskers andthe bending strength. The bending strength shown in FIG. 20 is obtainedby conducting the experiment shown in the first embodiment by usingsilicon nitride whiskers or silicon carbide whiskers as a ceramicmaterial. The lines in FIG. 20 are approximate curves drawn using themethod of least squares. When conducting this experiment, bendingsamples were formed with the respective additions, as shown in Table 21below, and their bending strengths were measured. TABLE 21 CompositionBending Bending Composition wt % Load N Strength MPa pure KCl 0 186.2551.55 pure KCl 0 250.024 2.08 pure KCl 0 226.274 1.89 pure KCl 0 308.7252.57 pure KCl 0 225.850 1.88 KCl + 5% SiC Whisker 5 718.75 5.99 KCl + 7%SiC Whisker 7 673 5.61 KCl + 5% SiC Whisker 5 581 4.84 KCl + 7% SiCWhisker 7 900 7.50 pure KCl 0 186.255 1.55 pure KCl 0 250.024 2.08 pureKCl 0 226.274 1.89 pure KCl 0 308.725 2.57 pure KCl 0 225.850 1.88 KCl +5% Si₃N₄ Whisker 5 721.25 6.01 KCl + 5% Si₃N₄ Whisker 5 640 5.33 KCl +5% Si₃N₄ Whisker 7 881.025 7.34 KCl + 5% Si₃N₄ Whisker 7 975.799 8.13

When silicon borate whiskers or silicon carbide whiskers were to be usedas a ceramic material in this manner, as shown in FIG. 20, if theaddition was 7%, the bending strength became higher than 8 MPa.

Therefore, when silicon borate whiskers or silicon carbide whiskers areused as a ceramic material, as described above, the same effect as thatobtained when the first embodiment is employed can be obtained.

11th Embodiment

When potassium hexatitanate whiskers or potassium octatitanate whiskerswere mixed in potassium chloride, a bending strength as shown in FIG. 21was obtained.

FIG. 21 is a graph showing the relationship between the addition ofpotassium hexatitanate whiskers or the addition of potassiumoctatitanate whiskers and the bending strength. The bending strengthshown in FIG. 21 is obtained by conducting the experiment shown in thefirst embodiment by using potassium hexatitanate whiskers or potassiumoctatitanate whiskers as a ceramic material. The lines in FIG. 21 areapproximate curves drawn using the method of least squares. Whenconducting this experiment, bending samples were formed with therespective additions, as shown in Table 22 below, and their bendingstrengths were measured. TABLE 22 Composition Bending BendingComposition wt % Load N Strength MPa pure KCl 0 186.255 1.55 pure KCl 0250.024 2.08 pure KCl 0 226.274 1.89 pure KCl 0 308.725 2.57 pure KCl 0255.850 1.88 KCl + 5% K₂O.6TiO₂ 5 661 5.51 KCl + 5% K₂O.6TiO₂ 5 856 7.13KCl + 5% K₂O.6TiO₂ 5 976 8.13 KCl + 5% K₂O.6TiO₂ 5 799 6.66 KCl + 5%K₂O.6TiO₂ 5 900 7.50 KCl + 7% K₂O.6TiO₂ 7 1140.5 9.50 KCl + 7% K₂O.6TiO₂7 905.2 7.54 KCl + 7% K₂O.6TiO₂ 7 778.7 6.49 KCl + 7% K₂O.6TiO₂ 7 10829.02 KCl + 7% K₂O.6TiO₂ 7 972.474 8.10 KCl + 7% K₂O.6TiO₂ 7 870.25 7.25KCl + 7% K₂O.6TiO₂ 7 1134.25 9.45 KCl + 7.05% K₂O.6TiO₂ 7.05 1052.4 8.77KCl + 7.05% K₂O.6TiO₂ 7.05 952.375 7.94 KCl + 8% K₂O.6TiO₂ 8 997.75 8.31KCl + 8% K₂O.6TiO₂ 8 557.7 4.65 KCl + 8% K₂O.6TiO₂ 8 1019 8.49 KCl + 8%K₂O.6TiO₂ 8 922.7 7.69 KCl + 8% K₂O.6TiO₂ 8 578.875 4.82 KCl + 8%K₂O.6TiO₂ 8 1048.72 8.74 KCl + 8% K₂O.6TiO₂ 8 646.15 5.38 pure KCl 0186.255 1.55 pure KCl 0 250.024 2.08 pure KCl 0 226.274 1.89 pure KCl 0308.725 2.57 pure KCl 0 255.850 1.88 KCl + 5% K₂O.8TiO₂ 5 715 5.96 KCl +5% K₂O.8TiO₂ 5 697 5.81 KCl + 5% K₂O.8TiO₂ 5 555 4.63 KCl + 5% K₂O.8TiO₂5 909 7.58 KCl + 5% K₂O.8TiO₂ 5 761 6.34 KCl + 5% K₂O.8TiO₂ 5 794.256.62 KCl + 7% K₂O.8TiO₂ 7 1088 9.07 KCl + 7% K₂O.8TiO₂ 7 993.599 8.28KCl + 7% K₂O.8TiO₂ 7 1350 11.25 KCl + 8% K₂O.8TiO₂ 8 1079.5 9.00 KCl +8% K₂O.8TiO₂ 8 1163 9.69 KCl + 8% K₂O.8TiO₂ 8 1188.25 9.90 KCl + 8%K₂O.8TiO₂ 8 1182 9.85 KCl + 8% K₂O.8TiO₂ 8 1175.77 9.80

When potassium hexatitanate whiskers or potassium octatitanate whiskerswere to be used as a ceramic material in this manner, as shown in FIG.21, if the addition was 7%, the bending strength became higher than 8MPa.

Therefore, when potassium hexatitanate whiskers or potassiumoctatitanate whiskers are used as a ceramic material, as describedabove, the same effect as that obtained when the first embodiment isemployed can be obtained.

12th Embodiment

When zinc oxide whiskers were mixed in potassium chloride, a bendingstrength as shown in FIG. 22 was obtained.

FIG. 22 is a graph showing the relationship between the addition of zincoxide whiskers and the bending strength. The bending strength shown inFIG. 22 is obtained by conducting the experiment shown in the firstembodiment by using zinc oxide whiskers as a ceramic material. The linein FIG. 22 is an approximate curve drawn using the method of leastsquares. When conducting this experiment, bending samples were formedwith the respective additions, as shown in Table 23 below, and theirbending strengths were measured. TABLE 23 Bending Composition BendingStrength Composition wt % Load N MPa pure KCl 0 186.255 1.55 pure KCl 0250.024 2.08 pure KCl 0 226.274 1.89 pure KCl 0 308.725 2.57 pure KCl 0225.850 1.88 KCl + 5% ZnO Whisker 5 401.45 3.35 KCl + 5% ZnO Whisker 5487.35 4.06 KCl + 10% ZnO Whisker 10 654 5.45 KCl + 10% ZnO Whisker 10510.899 4.26 KCl + 15% ZnO Whisker 15 612.75 5.11 KCl + 15% ZnO Whisker15 532.375 4.44

When zinc oxide whiskers are to be used as a ceramic material in thismanner, as shown in FIG. 22, if the addition is 15%, a salt core with ahigh bending strength can be formed.

Therefore, when zinc oxide whiskers are used as a ceramic material, asdescribed above, the same effect as that obtained when the firstembodiment is employed can be obtained.

FIG. 23 is a graph showing the relationship between the addition of eachof all the whiskers shown in the ninth to 12th embodiments describedabove and the bending strength. As is apparent from FIG. 23, of thewhiskers described above, the one that could form a salt core with thehighest bending strength was aluminum borate whiskers.

The relationship between the additions of the respective ceramicwhiskers and the fluidities were as shown in FIG. 24. The result of FIG.24 was obtained by an experiment of placing the ceramic whiskers andpotassium chloride in a Tammann tube, dissolving the mixture at 800° C.,stirring the mixture sufficiently, and reversing the Tammann tube upsidedown. Of the mixtures, one the melt of which flowed out from the Tammanntube was determined as “with fluidity” and one the melt of which did notwas determined as “without fluidity”.

The respective embodiments described above exemplified cases whereinpotassium chloride was used as a salt material. Other than potassiumchloride, a sodium chloride, or any one of a bromide, carbonate, andsulfate of potassium or sodium can be used as a salt material. As thesodium chloride, sodium chloride (NaCl) can be used. As the bromide ofpotassium or sodium, potassium bromide (KBr) or sodium bromide (NaBr)can be used. As the carbonate, sodium carbonate (Na₂CO₂) and potassiumcarbonate (K₂CO₃) can be used. As the sulfate, potassium sulfate (K₂SO₄)can be used.

13th Embodiment

When potassium bromide or sodium bromide was used as a salt material andaluminum borate whiskers were mixed in the salt material, a bendingstrength as shown in FIG. 25 was obtained.

FIG. 25 is a graph showing the relationship between the addition ofaluminum borate whiskers in potassium bromide or sodium bromide and thebending strength. FIG. 25 also describes the bending strength obtainedwhen aluminum borate whiskers are mixed in a different salt material. Asthe different salt material, potassium chloride and sodium chloride wereemployed. FIG. 25 describes a density ρ of each salt material in a solidstate. A density ρ of potassium bromide in the solid state is 2.75g/cm³. A density ρ of sodium bromide in a solid state is 3.21 g/cm³. Adensity ρ of potassium chloride in a solid state is 1.98 g/cm³. Adensity ρ of sodium chloride in a solid state is 2.17 g/cm³.

The bending strength shown in FIG. 25 is obtained by conducting theexperiment shown in the first embodiment by using aluminum boratewhiskers as a ceramic material. The lines in FIG. 25 are approximatecurves drawn using the method of least squares. When conducting thisexperiment, bending samples were formed with the respective additions,as shown in Tables 24 to 27 below, and their bending strengths weremeasured. Table 24 shows the bending strength obtained when aluminumborate is mixed in potassium bromide, and Table 25 shows the bendingstrength obtained when aluminum borate is mixed in sodium bromide.

Table 26 shows the bending strength obtained when aluminum borate ismixed in potassium chloride. Table 26 is obtained by adding the resultsof two experiments, that is, a case wherein the addition of aluminumborate whiskers is 0 and a case wherein the addition of aluminum boratewhiskers is 3 wt %, to Table 20. Table 27 shows the bending strengthobtained when aluminum borate is mixed in sodium chloride.

The type of aluminum borate whiskers employed in practicing thisembodiment is identical to that described in the ninth embodiment (seeFIG. 19 and Table 19). TABLE 24 Bending Composition Bending StrengthComposition wt % Load N MPa KBr 0 296.45 2.47 KBr + 3% Albolex M20 31735.25 14.46 KBr + 3% Albolex M20 3 1197.82 9.98 KBr + 3% Albolex M20 31206.42 10.05 KBr + 3% Albolex M20 3 1291.00 10.76 KBr + 3% Albolex M203 1389.52 11.58 KBr + 5% Albolex M20 5 1845.25 15.38 KBr + 10% AlbolexM20 10 2715.50 22.63 KBr + 12% Albolex M20 12 3304.75 27.54

TABLE 25 Bending Composition Bending Strength Composition wt % Load NMPa NaBr 0 227.20 1.89 NaBr + 3% Albolex M20 3 1210.75 10.09 NaBr + 3%Albolex M20 3 1424.50 11.87 NaBr + 3% Albolex M20 3 1527.07 12.73 NaBr +3% Albolex M20 3 2041.42 17.01 NaBr + 5% Albolex M20 5 2098.85 17.49NaBr + 8% Albolex M20 8 2531.25 21.09 NaBr + 10% Albolex M20 10 2554.4021.29

TABLE 26 Bending Composition Bending Strength Composition wt % Load NMPa KCl 0 186.255 1.55 KCl 0 250.024 2.08 KCl 0 226.274 1.89 KCl 0308.725 2.57 KCl 0 225.850 1.88 KCl 0 214.600 1.79 KCl 3 748.000 6.23KCl + 10% Albolex M20 10 2485.75 20.71 KCl + 10% Albolex M20 10 2466.7520.56 KCl + 10% Albolex M20 10 2488.75 20.74 KCl + 10% Albolex M20 102832.25 23.60 KCl + 10% Albolex M20 10 2262.89 18.86 KCl + 10% AlbolexM20 10 2758.00 22.98 KCl + 10% Albolex M20 10 2624.75 21.87 KCl + 10%Albolex M20 10 2155.35 17.96 KCl + 15% Albolex M20 15 4101.05 34.18KCl + 15% Albolex M20 15 3722.75 31.02 KCl + 15% Albolex M20 15 3763.5031.36 KCl + 15% Albolex M20 15 3973.75 33.11 KCl + 15% Albolex M20 153305.72 27.55 KCl + 15% Albolex M20 15 3783.02 31.53 KCl + 15% AlbolexM20 15 3411.75 28.43 KCl + 18.7% Albolex M20 18.7 4346.25 36.22

TABLE 27 Bending Composition Bending Strength Composition wt % Load NMPa NaCl 0 319 2.66 NaCl 0 253 2.11 NaCl 0 413 3.44 NaCl + 3% AlbolexM20 3 285.825 2.38 NaCl + 3% Albolex M20 3 468.95 3.91 NaCl + 3% AlbolexM20 3 429.924 3.58 NaCl + 5% Albolex M20 5 434.424 3.62 NaCl + 10%Albolex M20 10

When aluminum borate whiskers were to be mixed in potassium bromide orsodium bromide in this manner, the bending strength became higher than 8MPa if the addition was 3 wt % or more, as shown in FIG. 25. In FIG. 25,when aluminum borate whiskers are mixed in sodium chloride, a salt corewith a high bending strength can be formed.

Therefore, when potassium bromide or sodium bromide is used as a ceramicmaterial, as described above, the same effect as that obtained when thefirst embodiment is employed can be obtained.

As described above, in addition to use of chloride, bromide, or saltalone, as a salt material, a mixed salt of a potassium chloride orsodium chloride and a carbonate or sulfate of potassium or sodium can beused. For example, a mixed salt of potassium chloride and sodiumcarbonate, a mixed salt of sodium chloride and sodium carbonate, a mixedsalt of sodium chloride and potassium carbonate, or a mixed salt ofpotassium chloride and potassium sulfate can be used.

When a mixed salt is employed as a salt material in this manner, a saltcore with a low melting point can be formed, as is conventionally known.Therefore, the temperature required for casting the salt core can bedecreased. The power consumption of the casting device can be decreasedaccordingly, and the cost for manufacturing the salt core can bedecreased. When any one of the four types of mixed salts described abovewas used to form a salt core, unevenness did not readily form on thesurface of the cast core.

INDUSTRIAL APPLICABILITY

A core for use in casting according to the present invention is usefullyemployed in a mold for die-casting.

1. A core for use in casting which is formed by casting a mixed materialof a salt material and a ceramic material, said salt material comprisingany one of a chloride, a bromide, a carbonate, and a sulfate of any oneof potassium and sodium, and said ceramic material comprisingartificially synthesized granular one having a density falling within arange of 2.2 g/cm³ (exclusive) to 4 g/cm³ (inclusive).
 2. A core for usein casting according to claim 1, wherein said ceramic material comprisessynthetic mullite having a density of 2.79 g/cm³ to 3.15 g/cm³.
 3. Acore for use in casting according to claim 1, wherein said ceramicmaterial comprises aluminum borate having a density of 2.93 g/cm³.
 4. Acore for use in casting which is formed by casting a mixed material of asalt material and a ceramic material, said salt material comprising anyone of a chloride, a bromide, a carbonate, and a sulfate of any one ofpotassium and sodium, and said ceramic material comprising artificiallysynthesized granular one having a particle size of not more than 150 μm.5. A core for use in casting which is formed by casting a mixed materialof a salt material and a ceramic material, said salt material comprisingany one of a chloride, a bromide, a carbonate, and a sulfate of any oneof potassium and sodium, and said ceramic material comprising anygranular one of synthetic mullite, aluminum borate, boron carbide,silicon nitride, silicon carbide, aluminum nitride, aluminum titanate,cordierite, and alumina.
 6. A core for use in casting which is formed bycasting a mixed material of a salt material and a ceramic material, saidsalt material comprising any one of a chloride, a bromide, a carbonate,and a sulfate of any one of potassium and sodium, and said ceramicmaterial comprising whiskers of any one of aluminum borate, siliconnitride, silicon carbide, potassium hexatitanate, potassiumoctatitanate, and zinc oxide.
 7. A core for use in casting according toclaim 6, wherein said ceramic material comprises aluminum boratewhiskers.
 8. A core for use in casting which is formed by casting amixed material of a salt material and a ceramic material, said saltmaterial comprising a mixed salt obtained by adding any one of acarbonate and a sulfate of any one of potassium and sodium to a chlorideof any one of potassium and sodium, and said ceramic material comprisingartificially synthesized granular one having a density falling within arange of 2.2 g/cm³ (exclusive) to 4 g/cm³ (inclusive).
 9. A core for usein casting according to claim 8, wherein said ceramic material comprisessynthetic mullite having a density falling in a range of 2.79 g/cm³ to3.15 g/cm³.
 10. A core for use in casting according to claim 8, whereinsaid ceramic material comprises aluminum borate having a density of 2.93g/cm³.
 11. A core for use in casting which is formed by casting a mixedmaterial of a salt material and a ceramic material, said salt materialcomprising a mixed salt obtained by adding any one of a carbonate and asulfate of any one of potassium and sodium to a chloride of any one ofpotassium and sodium, and said ceramic material comprising artificiallysynthesized granular one having a particle size of not more than 150 μm.12. A core for use in casting which is formed by casting a mixedmaterial of a salt material and a ceramic material, said salt materialcomprising a mixed salt obtained by adding any one of a carbonate and asulfate of any one of potassium and sodium to a chloride of any one ofpotassium and sodium, and said ceramic material comprising any granularone of synthetic mullite, aluminum borate, boron carbide, siliconnitride, silicon carbide, aluminum nitride, aluminum titanate,cordierite, and alumina.
 13. A core for use in casting which is formedby casting a mixed material of a salt material and a ceramic material,said salt material comprising a mixed salt obtained by adding any one ofa carbonate and a sulfate of any one of potassium and sodium to achloride of any one of potassium and sodium, and said ceramic materialcomprising whiskers of any one of aluminum borate, silicon nitride,silicon carbide, potassium hexatitanate, potassium octatitanate, andzinc oxide.
 14. A core for use in casting according to claim 13, whereinsaid ceramic material comprises aluminum borate whiskers.
 15. A core foruse in casting according to any one of claims 8 to 14, wherein saidmixed salt is made from potassium chloride and sodium carbonate.